The present invention relates to an improved melt-spinning method and apparatus for synthetic fiber in which polymer melt being melt-spun is prevented from being thermally deteriorated and from forming impurities, and to an improved filter to be used for melt spinning of synthetic fiber. In particular, it relates to an improvement in the inner surface of a melt-spinning device.
Synthetic fiber of, for example, polyamide or polyester is melt-spun, and the melt-spun synthetic fiber is widely used in various fields of clothes and industrial materials as having good mechanical and chemical properties.
Apparatus for spinning polymer melt is generally composed of a device for melting polymer, a device for metering the resulting polymer melt, and a spinning pack for jetting out the thus-metered polymer melt, in which those devices are connected to each other via a pipe line. In the apparatus of that type, the polymer melt as fed into the spinning pack via the pipe line is distributed therein, then filtered and spun out through a spinneret. The spinning pack is composed of various members of a distributor, a filter, a pressure plate, a spinneret, etc. As the members constituting the melt-spinning device, generally used are those made of inexpensive stainless steel materials having well-balanced and good strength, corrosion resistance and workability. As the filter, generally used is a granular filter of, for example, silica sand, glass beads, alumina grains or stainless steel grains, either singly or as combined with a wire-netting filter. Recently, however, a plate filter made of non-woven fabric of fine metal fiber is being preferably used alone in place of the granular filter. The metal fiber constituting the plate filter is generally made of a stainless steel material. In ordinary melt-spinning apparatus for synthetic fiber that are popularly used at present, almost all the wall surfaces of the spinning device and other most members, with which the polymer melt being spun therethrough is contacted, are made of metal, and, naturally, the metal is in fact stainless steel except for limited exceptions such as sealant.
Stainless steel used for the pack members and filter is a well-balanced good material having various advantages noted above, and is difficult to substitute with any other materials. However, as so reported in Japanese Patent Publication (JP-B) No. Sho-53-29732, substances kept in contact with stainless steel members are often catalytically decomposed and deteriorated. JP-B Sho-53-29732 discloses the details of the catalytic action of the stainless steel members of a melt-spinning apparatus on the polymer melt of polyester or polyamide 66 being melt-spun therethrough, which is to decompose and deteriorate the polymer melt. The gel as formed through the decomposition and deterioration of the polymer melt causes various troubles such as yarn breaking or fuzz in the spinning and drawing step.
For polyamide fiber to be used in industrial materials, copper salts and/or various antioxidants are added to the polymer melt to be spun, which are for the purpose of improving the heat resistance of the fiber.
Though being effective in improving the heat resistance of polyamide fiber, copper salts added to polyamide melt are problematic in that they form copper compounds and metal copper that are insoluble in polyamide melt due to the high-temperature heat history applied to the melt being spun. The thus-formed copper compounds and metal copper precipitate and deposit on the surface of the wall of the flow duct in the spinning device thereby clogging the duct, or precipitate and deposit on the filter thereby increasing the pressure loss in filtration therethrough and shortening the exchange cycle of the spinning pack, or they penetrate into the spun fiber thereby causing various troubles such as yarn breaking or fuzz in the spinning and drawing step, and even lowering the process stability in the post-processing steps for, for example, warping, twisting and dipping the spun fiber after the spinning and drawing step. As so reported in Japanese Patent Application Laid-Open (JP-A) No. Hei-1-207417, it is known that, when polyamide melt containing a copper salt as the stabilizer is kept in contact with a metal member made essentially of iron, for example, with a stainless steel member in a spinning device, the formation of insoluble copper compounds is accelerated due to the electrochemical reaction between the copper ions and the iron component in the member. For this reason, it is unfavorable to use a stainless steel material in forming the wall of the melt-spinning device to be contacted with the polymer melt being spun therethrough.
The troubles to be caused by the contact between the polymer melt and the stainless steel or the like metal member in the spinning device may occur throughout the entire region of the polymer melt pathway, but they occur noticeably around the filter, especially the plate filter made of stainless steel fiber having a large contact area with polymer melt, as so reported in JP-B Sho-53-29732, and also JP-A Hei-7-268715 and Hei-7-268716.
In order to evade the troubles in melt spinning noted above, proposed is a means of using a high-chromium alloy in the filter part having a large contact area with polymer melt, in JP-B Sho-53-29732. Also in JP-A Hei-1-207417, it is written that a high-chromium alloy is effective in preventing the precipitation of copper compounds from polyamide that contains copper as the antioxidant while the polyamide melt is spun. However, the high-chromium alloy is expensive and is poorly workable, and it is extremely difficult to form the alloy into a plate filter of non-woven fabric of the alloy fiber to be favorably used in melt spinning of polymer melt. Even if chromium may be plated on the surface of members to thereby increase the chromium density in their surface, the plating is not applicable to members having a complicated shape.
Apart from surface modification with chromium, JP-A Hei-6-101119 discloses the use of SiO.sub.2, Al.sub.2 O.sub.3, TiC or the like as the material of the surface of members to be contacted with pitch melt to be melt-spun. However, pitch is a low-molecular polymer naturally containing many impurities in large quantities, while as compared with this, polymers having a high degree of polymerization, such as polyester and nylon, have a lower impurity content and are more stable. JP-A Hei-6-101119 suggests nothing about the effect of the surface material used therein in melt spinning of such high-molecular polymers.
JP-B Hei-5-32485, Hei-5-32486 and Hei-5-32487 disclose a method of melt-spinning synthetic fiber through a granular filter made of alumina grains, stainless steel grains or the like, or through a combination of the granular filter and a plate filter, in which is both the granular filter and the plate filter optionally combined with it are coated with a modified silicone film. However, these are silent on any other film-forming substances except modified silicone, and has no disclosure relating to the technique of melt-spinning nylon that contains a copper compound as the antioxidant and even the technique of preventing the precipitation of the antioxidant from nylon being spun. In these patent publications, only the deterioration of polymer on the surface of the granular filter is discussed, but nothing is suggested therein relating to the essential effect of the present invention which is directed to preventing the formation of impurities on the surface of a filter, especially a metal fiber filter, to thereby prevent the pressure loss in filtration through the filter from being increased and to improving the capability of the filter.
The troubles to be caused by the contact between metal members such as typically stainless steel members and polymer melt noted above are frequently seen especially in melt spinning of polymer melt, but are not limited to only the case of melt spinning operation. The troubles in question are common to all shaping techniques for forming articles from resin melt, for example, for forming films from resin melt, for molding shaped articles from resin melt and even for forming pellets from resin melt.
As has been mentioned hereinabove, stainless steel materials have the advantage of high strength, good workability and low cost, but have the disadvantage of promoting the deterioration of polymer as contacted therewith. Therefore, it is desired not to use any special and expensive materials, except for stainless steel materials, to construct devices for shaping polymer melt. In particular, it is desired to carry out melt-spinning of polymer melt, while using inexpensive melt-spinning devices not having any negative influences on the polymer melt being spun therethrough.